Phase-shift keying (PSK) is a digital modulation technique that conveys data by changing (i.e. modulating) the phase of a carrier signal. Essentially, binary digits (bits) are encoded by associating a discrete set of phases of the carrier signal with a particular pattern of bits, known as a symbol. In differential phase-shift keying (DPSK) it is the change in successive phases of the signal that is used to determine the bit pattern, rather than the actual phase of the signal at any point in time.
Binary phase-shift keying (BPSK) makes use of two distinct phases separated by 180°. However, this technique only allows for 1 bit to be encoded per symbol (i.e. providing a total of 2 bits for each complete wavelength) and so it is not suitable for high data-rate applications. Quadrature phase-shift keying (QPSK) employs four discrete phases and can be used to encode two bits per symbol by combining an in-phase wave (denoting 4 discrete bits—one per quarter wavelength—representing the first bits in each symbol) and a quadrature-phase wave, having a phase-shift of a quarter of a wavelength with respect to the in-phase wave (the quadrature-phase wave denoting a further 4 discrete bits, representing the second bits in each symbol). Thus, QPSK can be used to significantly increase a data rate when compared to BPSK. Both BPSK and QPSK can be implemented using differential PSK to form differential BPSK (DBPSK) and differential QPSK (DQPSK), respectively.
Advanced modulation formats are increasingly important in optical communications due to their ability to provide increased spectral efficiency, higher receiver sensitivity, and better tolerance to chromatic dispersion and nonlinear effects. In particular, return-to-zero (RZ) DQPSK has been shown to provide quite high system performance results.
FIG. 1 illustrates schematically an example of a RZ-DQPSK transmitter 100. The transmitter comprises a source 101 (e.g. a laser diode), a DQPSK modulator 102 and a pulse carver 103 driven by a clock source 110. The DQPSK modulator 102 comprises an input splitter 104 feeding two phase modulators 105, 106 arranged in parallel, and an output coupler 107. The phase modulators 105, 106 can be provided by two nested Mach-Zehnder Modulators (MZM). One arm of the DQPSK modulator 102 is also provided with a further quadrature phase modulator 108. The source 101 generates an optical carrier wave which is split by the splitter 104 and equally distributed to the two phase modulators 105, 106. Each of the phase modulators 105, 106 is driven by one of two binary drive signals generated by a pre-coder 109. The pre-coder 109 converts the data streams that are to be encoded into the relevant in-phase (I) and quadrature (Q) phase drive signals, with one of the two phase modulaltors being driven by the I drive signal whilst the other is driven by the Q drive signal. The quadrature phase modulator 108 then introduces a π/2 (90°) phase shift between the two optical signals, which puts them in quadrature to each other (i.e. such that they form separate I and Q components). The two optical signals are then combined in the output coupler 107 resulting in one of the four phase shifted symbols (i.e. π/4, 3π/4, 5π/4 and 7π/4) of a non-return-to-zero (NRZ) DQPSK signal.
The pulse carver 103 is then used to produce a RZ-DQPSK signal by carving pulses out from the NRZ-DQPSK signal. The pulse carver 103 could also be placed before the DQPSK modulator. Typically, advanced RZ modulation requires that the pulse carver be implemented by a MZM that is driven by a clock source providing sinusoidal electrical clock signals. For example, 50% duty cycle RZ can be generated using a sinusoidal clock signals with a peak-to-peak amplitude of Vπ, a frequency corresponding to the symbol rate/data rate (i.e. baud-rate clock) and a phase offset of −π/2 radians (−90°).
For a dual-drive MZM pulse carver it is important that the two voltages driving the two arms are of equal power and of anti-phase. However, the imperfect nature of the transmission structures (i.e. RF cable, connector and PCB tracks), the amplifiers, and the source of the driving voltages leads to the generation of power imbalance and skew (i.e. temporal misalignment of the drive waveform) between the two driving voltages. In an attempt to mitigate these problems, transmitter circuits often employ a differential amplifier, or an amplifier on each arm of an MZM pulse carver. However, the inclusion of such active devices within a circuit is costly.